Device for automatic thickness control of rolled strips

ABSTRACT

A device for automatic thickness control of rolled strips which ensures an accuracy of the rolled material from + OR - 0.002 mm to + OR - 0.003 mm, does not call for major capital expenses and long stops of the rolling mills for its introduction and is characterized by high operating reliability and simple servicing. The device for automatic thickness control of rolled strips in installations for the manufacture of strip materials, comprising symmetrically located on both sides of the roll stand at least one hydraulic power cylinder installed between the supports of the working rolls, a damping hydraulic cylinder whose piston is fixed while the strip is passing through the roll stand, and a controllable valve connected so that the hydraulic system formed by said hydraulic cylinder and valve is closed and creates a local increase in the rigidity of the roll stand within the range of forces which is approximately equal to the range of variations of the rolling pressure from the preset value. The device has a positive feedback circuit actuated by the fluid pressure in the closed hydraulic system, this circuit comprising a pressure meter in the hydraulic power cylinder, an element for comparing the measured deviation of pressure with the value proportional to the preset coefficient of positive feedback, connected with the pressure meter, and an actuating element connected to said comparison element and to the correcting element made in the form of a hydraulic cylinder communicating with the hydraulic power cylinder.

United States Patent Sokolov et al.

DEVICE FOR AUTOMATIC THICKNESS CONTROL OF ROLLED STRIPS Inventors: Evgeny Vyacheslavovich Sokolov,

Filed:

March 4, 1971 Appl. No.: 120,845

US. Cl. ..72/6, 72/28, 72/20,

Int. Cl. ..B2lb 37/00 Field of Search ..72/8, 19, 20, 28, 6

References Cited UNITED STATES PATENTS 2/1971 11/1970 Guillot ..72/8 10/1970 Sims ..72/8

Rastelli ..72/19 [1 1 3,686,907 1 Aug. 29, 1972 Primary Examiner-Milton S. Mehr Att0rney-Waters, Roditi, Schwartz & Nissen [57] ABSTRACT A device for automatic thickness control of rolled strips which ensures an accuracy of the rolled material from $0.002 mm to 10.003 mm, does not call for major capital expenses and long stops of the rolling mills for its introduction and is characterized by high operating reliability and simple servicing.

The device for automatic thickness control of rolled strips in installations for the manufacture of strip materials, comprising symmetrically located on both sides of the roll stand at least one hydraulic power cylinder installed between the supports of the working rolls, a damping hydraulic cylinder whose piston is fixed while the strip is passing through the roll stand, and a controllable valve connected so that the hydraulic system formed by said hydraulic cylinder and valve is closed and creates a local increase in the rigidity of the roll stand within the range of forces which is ap proximately equal to the range of variations of the rolling pressure from the preset value. The device has a positive feedback circuit actuated by the fluid pressure in the closed hydraulic system, this circuit comprising a pressure meter in the hydraulic power cylinder, an element for comparing the measured deviation of pressure with the value proportional to the preset coefficient of positive feedback, connected with the pressure meter, and an actuating element l d. tasaislsaneatisea e t and to th m recting element made in the form of a hydraulic cylinder communicating with the hydraulic power cylinder.

9 Claims, 3 Drawing Figures DEVICE FOR AUTOMATIC THICKNESS CONTROL OF ROLLED STRIPS The present invention relates to devices for automatic thickness control of rolled strips and can be employed in installations for rolling such strip materials as paper, synthetic films and mainly in the mills for hot and cold rolling of ferrous and nonferrous metals.

Known in the art are devices for automatic thickness control of rolled strips in the installations for the manufacture of strip materials. Some of these devices incorporate an actuating mechanism in the form of an electrical (or hydraulic) drive of the pressure screws or hydraulic jacks thrusting apart the supports of the working rolls and a measuring element in the form of dynamometers which measure the full rolling pressure. These devices utilize the relationship between the thickness of the rolled strip, rolling pressure, initial 4 opening between the working rolls determined by the position of the pressure screws, and the rigidity of the roll stand. These devices have the following disadvantages:

a. necessity for accurate measurement of the full rolling pressure;

b. impossibility of correcting the thickness of the rolled strip by the run-out of the back-up rolls;

c. necessity for taking in account the variations in the height and rigidity of the oil wedge in the liquid-friction bearings of the back-up rolls caused by changes in the rolling speeds;

d. necessity for fixing accurately the position of the pressure screws.

The above disadvantages limit the accuracy of thickness control of, say, a hot-rolled steel strip to 005-003 mm. However, for reaching this limit, a number of certain technical difficulties must be overcome; in particular, it is necessary to provide a quickresponse actuating mechanism (electrical drive of pres sure screws, hydraulic jacks, etc.) and ensure the requisite accuracy in the channel for measuring the full rolling pressure. These difficulties become particularly great if the run-out of the back-up rolls in the roll stand reaches 0.05 mm and over.

Also known in the art are the devices for automatic thickness control of rolled strips which are based on the method of increasing the rigidity of the roll stand by prestressing it (see, for example, French Pat. No. 1,365,124, and the rolling mill produced by SKF, Sweden). These devices comprise special pressure screws between the supports of the back-up rolls, said screws used as prestressed metal pads in the process of correcting the deviations of the strip thickness from the desired value. These devices are more advantageous than the former since they utilize relatively simple means for increasing the rigidity of the stand by 40-50 percent, which allows the variations inthe thickness of a hot-rolled steel strip to be reduced to i 0.12-0.09 mm.

Besides, these devices provide for flexible control of the stand rigidity, from a non-rigid stand to a highly rigid stand, by the introduction of full rolling pressure meters and a powerful hydraulic system which develops forces higher than the maximum rolling pressures. Therefore, for this category of devices the.reduction of the thickness control error to i 0.05-0.03 mm involves the same difficulties as those inherentin the devices discussed above.

Another known device for automatic thickness control of rolled strips is based on a local increase in the rigidity of the roll stand, i.e., increasing its rigidity only in a relatively narrow range of variations in the rolling pressure (see USSR Inventors Certificate No. 206499, Cl. 7a, I). This device comprises hydraulic power cylinders installed symmetrically between the supports of the mill rolls on both sides of the stand, damping cylinders whose pistons are fixed while the strip is passing through the roll stand, and controllable valves connected in such a manner that they form a closed and sufficiently rigid hydraulic system.

This ensures a material increase in the rigidity of the stand by prestressing; however, the force of stand prestressing is in this case considerably smaller than the force of rolling pressure.

The principle of locality in the given method makes it possible to directly increase the rigidity of the working roll opening and to correct the effect produced by the run-out of the back-up rolls on the: thickness of the strip and the influence of variations in the height and rigidity of the oil wedge in the back-up roll bearings caused by changes in the rolling speed. In addition, the principle of locality in increasing the rigidity of the stand makes it possible to dispense with the dynamometers measuring the full rolling pressure. This device reduces considerably the requirements for the electric (or hydraulic) drive of the pressure screws so that the latter operate under the conditions approaching those prevailing in non-automated rolling mills.

The preset thickness of the strip at the outlet end of the roll stand is ensured by controlling the volume of the liquid in a closed hydraulic system as a function of, say, a signal sent by a strip thickness meter installed at the output end of the mill. Lagging of the correcting signal may be neglected in the given case since the volume of the liquid in the closed hydraulic system remains virtually unchanged within the period required for the strip to pass from the stand to the meter.

An object of the present invention is to provide a device for automatic thickness control of rolled strips, which will provide for obtaining; the rolled material with the accuracy of from i 0.002 to :t 0.003 mm.

Another object of the present invention is to provide a device whose realization will not call for considerable capital expenses and prolonged stops of the rolling mills.

Still further object of the present invention is to provide a reliable device.

The present invention consists in a device for automatic thickness control of rolled strips in installations for the manufacture of strip materials, comprising at least one hydraulic power cylinder installed between the supports of the working rolls, another hydraulic cylinder serving as a damping mechanism whose piston is fixed while the strip is passing through the roll stand, these cylinders being arranged symmetrically on both sides of the roll stand, and a controllable valve connected in such a manner that the hydraulic system formed by said hydraulic cylinders and the valve is closed and provides for a local increase in the rigidity of the roll stand within the range of forces which is approximately equal to the range of deviations of the rolling pressure from the preset value, which device, according to the invention, has a positive feedback channel actuated by the liquid pressure in the closed hydraulic system comprising a pressure meter for measuring pressure in the space of the hydraulic power cylinder, a comparison element which compares the measure deviation of pressure with the value proportional to the preset coefficient of positive feedback, is connected with the pressure meter, and an actuating element connected to said comparison element and to a correcting element made in the form of a hydraulic cylinder communicating hydraulically with the hydraulic power cylinder.

This increases rigidity in a local section of the elastic deformation characteristic of the stand to the value which is sufficient for controlling the thickness of the rolled strip along its length accurate within 0.0050.003 mm.

It is practicable that the positive feedback channel should be made hydromechanical in which the inner space of the hydraulic cylinder which functions as an actuating mechanism is communicated hydraulically with the space of the control valve whose movable piston is kinematically linked at one end with the piston of the hydraulic cylinder used as a pressure meter while its other end is connected to a flexible element which is linked mechanically with the piston of the correcting hydraulic cylinder.

The positive feedback channel may be of the electromechanical type in which the pressure meter is connected via a divider which serves as a setter of the positive feedback coefficient and via the comparison element with the actuating element whose function is fulfilled by an electromechanical means which converts electric voltage into motion with negative motion feedback, said means being connected with the piston of the correcting hydraulic cylinder.

For limiting the range of pressure fluctuations in the hydraulic power cylinders which result from the process of correction of the strip thickness variations along its length to 1-2 percent of the preset pressure for ensuring stable transverse profile of the strip, the device for automatic thickness control of rolled strips comprises a channel for controlling the thrust of working rolls which stabilizes the thrust with relation to the preset value.

The channel for controlling the thrust of the working rolls is capable of controlling either the pressure in the hydraulic cylinders installed between each pair of supports of the back-up rolls, or the electric drive of the pressure screws.

It is practicable that the roll thrust control channel should comprise a setter of working roll thrust, a separate pressure meter in the space of the hydraulic power cylinder and a control valve whose movable piston should be connected at one end with said pressure meter while its other end should be connected to the working roll thrust setter, the control valve space being in hydraulic communication with the hydraulic cylinder installed between the supports of the back-up rolls.

The working roll thrust control channel may comprise a correcting element in the form of a hydraulic cylinder communicating with the hydraulic cylinder installed between the supports of the back-up rolls, and an electromechanical means for converting electric voltage into motion, said means being mechanically linked with the piston of the correcting hydraulic cylinder and electrically connected with the pressure meter of the positive feedback channel.

To eliminate the limitations in the accuracy of strip thickness control, the device may comprise a channel for programmed correction of working roll machining inaccuracy, this channel comprising a memory unit for memorizing the pressure changes in the hydraulic power cylinder as a function of the working roll turning angle, a unit for measuring the current value of the working roll turning angle, an electromechanical means for converting electric voltage into motion which is electrically connected with the above-mentioned units and mechanically linked with the piston of its own correcting hydraulic cylinder, the latter being in hydraulic communication with the hydraulic power cylinder, and a control unit which synchronizes the effect produced by said units on the electromechanical means which converts electrical voltage into motion.

Other objects and advantages of the present invention will become more apparent from the description of its embodiments given by way of example with reference to the accompanying drawings, in which:

FIG. 1 shows a device for automatic thickness control of rolled strips, according to the invention;

FIG. 2 shows the same device in which the channel of positive feedback and the channel for controlling the working roll thrust are of the electromechanical type, according to the invention;

FIG. 3 shows a channel for programmed correction of working roll machining inaccuracy, according to the invention.

Now the device for automatic thickness control of strips in hot-rolling mills will be described in more detail by way of example.

Installed in a bed 1 (FIG. 1) of the rolls stand are pads 2 used as supports for back-up rolls, pads 3 supporting working rolls, and pressure screws 4 with an electric motor 5.

According to the invention, the device comprises a hydraulic power cylinder 6 with a piston 7 at each side of the stand (the drawing shows the elements of the device and their interaction for one side of the stand only), said power cylinder 6 being installed between the supports of the working rolls in such a manner that, being filled with fluid under pressure, it would thrust apart these supports. The device also comprises a damping hydraulic cylinder 8 whose piston 9 is fixed in the extreme position by a spring 10 in the course of the strip rolling. The initial tension of the spring 10 determines the value of pressure corresponding to the beginning of the damping action. The device also comprises a controllable valve 11.

Said hydraulic cylinders 6 and 8 and the valve 11 are connected so that they form a closed hydraulic system. According to the invention, the closed hydraulic system incorporates a channel of positive fluid pressure feedback. FIG. 1 shows a hydromechanical form of this channel. Here the actuating element in the form of a hydraulic cylinder 12 communicates with a control valve 13 whose movable piston 14 is kinematically linked at one end with piston 15 of a hydraulic cylinder 16 used as a pressure meter while its other end is connected with a flexible element 17'mechanically connected with piston 20 of a correcting hydraulic cylinder 21 via a lever 18 and piston 19 of the hydraulic cylinder 12. The lever 18 is mechanically connected to a movable support 22 which sets the relation between the arms of the lever 18.

Shown in FIG. 2 is a device wherein the positive feedback channel is of the electromechanical type. A pressure meter 23 consists of a magnetoanisotropic force transmitter and a piston 24 which converts fluid pressure into force and is connected to a comparator unit 25 in which the reference voltage is connected in opposition to the voltage of the pressure meter 23 via an autotransformer.

The comparator unit 25 is connected to a phase-sew sitive amplifier 26, for example, of the transistorized type, whose output is connected with a comparator unit 28 via a divider 27 made in the form of a potentiometer which sets the coefficient of the positive feedback. The output of the comparator unit 28 is electrically connected via an amplifier 29 with an electric motor 30 which is mechanically linked with a motion feedback transmitter 31 in the form of a potentiometer while the output of the latter is connected with the second input of the comparator unit 28. The electric motor 30 is mechanically linked by a screw-and-nut pair 32 with piston 33 of a correcting hydraulic cylinder 34.

For stabilizing the transverse profile of the strip, the device comprises a channel for controlling the working roll thrust, this channel being of the hydromechanical type. Piston 35 (FIG. 1) of a hydraulic cylinder 36 serving as a pressure meter in the closed hydraulic system is linked kinematically with one end of movable piston 37 of a control valve 38 while the other end of the piston 37 is mechanically coupled with the working roll thrust setter made in the form of a flexible element 39 with a mechanism 40 for controlling its tension. The space of the control valve 38 communicates with a hydraulic cylinder 41 installed between the supports of the backup rolls.

The working roll thrust control channel of the electromechanical type is illustrated in FIG. 2.

It comprises a correcting element in the form of a hydraulic cylinder 42 whose piston 43 is mechanically coupled with an electric motor 44. The hydraulic cylinder 42 communicates hydraulically with the hydraulic cylinder 41 installed between the supports of the back-up rolls.

The electric motor 44 is electrically connected with the output of the amplifier 26 in the positive feedback channel via an amplifier 45 of, say, transistorized design.

Shown in FIG. 3 is a channel from programmed correction of the working roll machining inaccuracy intended to eliminate the limitations in the accuracy of strip thickness control. This channel comprises a unit for measuring the current value of the working roll turning angle which consists of a pulse transmitter 46 of the working roll speed which sends a pulse marking the I beginning of each revolution and a pulse counter 47 with a converter of the number of pulses into a code, said converter employing, say, transistor flip-flops. The counter 47 is connected via a control unit 48 with a memory unit 49 which memorizes pressure changes in the hydraulic power cylinder 6 as a function of the turning angle of the working rolls, said control unit employing, say, ferrite-transistor cells.

Besides, the amplifier 26 of the positive feedback channel is connected to a converting device in the form of a voltage-code converter 50, connected with the control unit 48 which, in turn, is connected with an electromechanical means 53 converting electric voltage into motion, e.g., with a magnetostrictive converter, via the converting device in the form of a codevoltage" converter 51 and an amplifier 52 employing transistors with a controllable amplification factor.

The electromechanical means 53 is mechanically linked with piston 54 of a correcting hydraulic cylinder 55.

Let us consider the operating principle of the device for automatic thickness control of rolled strips by referring to the device illustrated in FIG. 1.

A local increase in the rigidity of the stand is achieved with the aid of a hydraulic system constituted by the hydraulic power cylinders '6, hydraulic cylinder 8 and the controllable valve 11. This hydraulic system will be referred to hereinafter as a roll stand prestressing circuit.

Let us take for an example a simplified method of setting the roll stand to adesired size of the rolled strip. In this case the controllable valve 11. is open and the fluid flows at a pressure P, from a pump (not shown in the drawing) into each hydraulic power cylinder 6. The pistons 7 of the hydraulic power cylinders develop forces which deform preliminarily the flexible elements of the roll stand, i.e., the bed, pressure mechanism, rolls, etc.

Using the pressure screws, the mill operator sets the required opening of the working rolls and feeds the strip into the stand. The strip adds to elastic deformation of the roll stand so that the opening of the working rolls increases correspondingly, while the pistons 7 of the hydraulic cylinders 6 symmetrically arranged in the stand move upward applying the original force to the stand.

If the mill operator considers that the strip discharged from the stand is of the required thickness and that the setting of the stand is completed, he closes the controlable valve 11 with the aid of a remote drive (not shown in the drawing). This closes the hydraulic system which becomes essentially rigid owing to but a slight deformation of the closed volume of fluid on changes of pressure in it. The piston 9 of the hydraulic cylinder 8 is fixed in the upper position by the force F which is built up by the spring 10 and which is approximately equal to 2 P this force being used to set the upper limit of pressure control in the hydraulic power cylinder 6.

As a result, within the above-mentioned pressure range said rigid hydraulic system transforms the hydraulic power cylinder 6 with the piston 7 into a rigid pad installed between the supports of the working rolls and prestressed with a force which is considerably smaller than the rolling pressure. If the pressure exerted by the rolled strip on the working rolls varies during rolling, the opening of these rolls also varies correspondingly, thus causing variations in the thickness of the strip discharged from the stand. However, provision of said rigid pads between the supports of the working rolls reduces materially the variations in the roll openings. For instance, when the rolling pressure increases and the opening of the working rolls becomes correspondingly larger, the forces created in the pads by prestressing are reduced with the increase in the rigidity of the pad, supports, and of the necks and barrels of the working and back-up rolls.

With a reduction in the rolling pressure the forces inthe pads grow which ensures automatic adjustment of the roll opening.

In an ideal case characterized by an infinite rigidity of said elements of the roll stand prestressing circuit and of said elements of the stand the variations in the opening of the working rolls during rolling would have been completely eliminated while the absolute valve of variations in the forces in said rigid pads would have been equal to that of the variations in the rolling pressure with an opposite sign.

However, the actual rigidity of the elements participating in prestressing the stand is such that the rigidity of the roll stand is increased due to prestressing by only 80-l00 percent in the section of the stand elastic deformation characteristic, this section being determined by the rolling pressure variation range which reduces the variations in the thickness of the rolled strip discharged from the mill only to i 0.08-0.05 mm depending on the rolling conditions.

At the moment when the rolled strip leaves the stand, the force in the rigid pads, i.e., the pressure in the hydraulic power cylinders 6 rises sharply and the piston 9 of the hydraulic cylinder 8 moves thus vacating the required space for the fluid forced out of the hydraulic power cylinders 6 at the moment when elastic deformation of the stand caused by the presence of the rolling pressure is relieved. As the next strip is fed into the stand a contrary process takes place: the piston 9 of the hydraulic cylinder 8 is again fixed in the uppermost position by the spring 10, pressure in the closed hydraulic system drops approximately to P while its deviations from this level, i.e., pressure increment AP are an indication of a beginning of automatic control.

In this case retention of the fluid volume in the closed hydraulic system is of cardinal importance both at the moment of strip rolling, at the moment of its discharge from the stand, and afterwards.

It follows from the above that the roll stand prestressing circuit employs the simplest and most economical means for limiting the deviations in the strip thickness to i 008-005 mm; besides, the device responds equally well to the variations in the parameters of the rolled strip, e.g. its temperature, and to the current variations in the parameters of the roll stand, e.g., changes in the height and rigidity of the oil wedge in the bearings of the back-up rolls or the run-out of the back-up rolls, except the variations in the parameters of the working rolls. However, for example, during hot rolling of steel strips it may become necessary to reduce the error in the strip thickness control to i 0.01 mm and less. The operating accuracy of the present device is increased with the aid of the positive fluid pressure feedback channel in the hydraulic system, this channel compensating for the above-mentioned influence of the flexible elements in the stand prestressing circuit on the accuracy of strip thickness control.-

Said positive feedback channel is intended for partial or complete compensation for the negative feedback effects of the flexible elements of the prestressing Cir cuit on the variations of pressure in the closed hydraulic system during automatic adjustment of working roll opening. If the controllable valve 11 is open during roll stand setting, the piston 20 of the correcting hydraulic cylinder 21 occupies a position which is determined by the value of pressure P,, and by the parameters of the mechanical linkage between the movable piston 14 of the control valve 13 and the piston 20 of the correcting hydraulic cylinder 21. This is caused by the fact that the movable piston 14 of the control valve 13 is in a state of equilibrium under the action of two forces: applied by the piston 15 of the hydraulic system pressure meter 16 and by the spring 17 whose tension depends on the position of the piston 20. For example, if the force applied by the piston 15 exceeds at a certain moment the force applied by the spring 17, the piston 14 will move to the right and pressure P will be fed to the hydraulic cylinder 12, moving the piston 20 to the left. As the piston 20 moves gradually to the left, the force of the spring 17 transmitted to the piston 14 grows until a state of equilibrium is reached at which the movable piston 14 comes again to the neutral position and the pressure fluid is no longer fed to the hydraulic cylinder 12.

The moment when said process is discontinued during rolling is determined, according to the invention, by setting the value of the positive feedback coefficient which is selected in the hydromechanical channel by setting the relation between the arms of the lever 18. To make the principle of this selection more apparent, let us consider the process of adjusting the positive feedback channel before rolling. For this purpose, the working rolls of the roll stand are pressed together by the pressure screws 4 with a force approaching the rolling pressure. This simulates the infinite rigidity of the opening of the working roll barrels because changes in the height of the elastically deformed section of the roll barrels within the range of rolling pressure variations may be neglected.

The position of the movable support 22 during setting is selected so that any motion of the piston 20 of the correcting hydraulic cylinder 21 would lead to an identical change in the forces applied to both sides of the movable piston 14 of the control valve 13 without changes in the opening of the working rolls in spite of the pressure changes in the closed hydraulic system.

Let us consider an example when the setting of the stand is completed and the controllable valve 11 closes at the moment of strip rolling for stabilizing the thickness of the strip at the stand output end (this thickness having been set by the mill operator).

Assume that the new section of the strip entering the stand has a higher temperature. Then there will be a pressure increment AP, in the closed hydraulic system relative to the initial value of P It shows, on the one hand, that the process of automatic control has ensured a certain value of deviation from the strip thickness while on the other hand it means that there is a static control error which is directly proportional to the value of AP and inversely proportional to the rigidity coefficient of the closed hydraulic system.

The positive feedback channel makes it possible to dispose of said static error. The pressure increment AP, causes the movable piston 14 of the control valve 13 to move to the right thereby shifting the piston 20 of the correcting hydraulic cylinder 21 to the left thus providing for a further rise of pressure in the closed hydraulic system.

However, this increases the force of the spring 17 and, incompliance with the above described adjustment of the positive feedback channel, this force grows quicker thandoes thepressure increment in the closed hydraulic system caused by the movement of the piston 20 because the roll opening in this case isnot rigid.

Then, if the piston 20 moves a certain distance AS from the position in which the controllable valve 1 1 has been closed and, after the pressure in the closed hydraulic system has increased by AP,', the piston 14 returns again to the neutral position whereas the movement of the piston 20 of the correcting hydraulic cylinder 21 ceases. This completes the process of positive feedback caused by a pressure increment in the closed hydraulic system.

Let us determine the exact end of motion of the piston 20 of the correcting hydraulic cylinder 21 caused by the pressure increment AP, in the hydraulic system, this increment resulting from the corresponding reduction of the working roll opening during rolling. As the piston 20 moves and the pressure in the hydraulic system rises, the opening of the working roll increases to the original value. This is accompanied by a reduction in the value of AP, as a component of the total pressure increment in the closed hydraulic system. At the moment when the opening of the working roll returns to the exact original value, AP, vanishes completely so that the resultant pressure increment AP,in the closed hydraulic system is caused only by the corresponding movement AS of the piston 20 of the correcting hydraulic cylinder 21 and in the given case this is also a condition for the retaking of the neutral position by the movable piston 14 of the control valve 13. In this case the resultant pressure increment AP, in the closed hydraulic system, which is always larger than AP,, is directly proportional to the movement AS of the piston 20:

AP, K -AS, where:

K proportionality factor denoting the rigidity of the closed hydraulic system.

In a general case K is the coefficient of the positive feedback of the channel.

As the pressure in the closed hydraulic system drops owing to an increased pressure exerted by the rolled strip on the working roll, the piston of the positive feedback channel moves to the right. In case of any pressure variations in the hydraulic system, the positive feedback channel ensures the above-mentioned relationship between the pressure increment AP, and the movement AS of the piston 20 of the correcting hydraulic cylinder 21.

Shown in FIG. 2 is the positive feedback channel of the electromechanical type. This channel functions as follows:

The pressure meter 23 of the closed hydraulic system sends a signal into the comparator unit 25 where the value of AP, of the current value of pressure is determined by comparing it with pressure P,. The amplifier 26 is phase-sensitive so that the sign AP, can be distinguished at its output. The divider 27 is set for a value of the positive feedback coefficient so that the D.C. volt age fed into the comparator unit 28 from the divider output is proportional to the relation AP,/K;

Simultaneously, the second input of the comparator unit 28 receives a signal from the transmitter 31 indicating the positionof the piston 33 of the correcting hydraulic cylinder 34. The signal which is proportional to the difference between AP,'/K and AS fed from the output of the comparator unit 28 via the amplifier 29 to the input of the electric motor 30 controls the movement of the piston 33 of the correcting hydraulic cylinder 34 so that (AP,'/K)AS 0 or AP, K 'AS. In the electromechanical positive feedback channel the coefficient of positive feedback can be set more flexibly, for example by a controlling computer. For the same purpose the hydrornechanical positive feedback channel must be provided with a remote control for moving the movable support 22.

Increasing the rigidity on the local section of the stand elastic deformation characteristic and reducing the error of strip thickness control along the strip length, the positive feedback channel simultaneously increases the amplitude of pressure variations in the closed hydraulic system. For stabilizing the transverse profile of the rolled strip, according to the invention,

the stand prestressing circuit is connected to a channel controlling the thrust of the working rolls.

The movable piston 37 of the control valve 38 of this channel is kept in equilibrium by two forces: from the piston 35 of the hydraulic cylinder 36 and from the flexible element 39. The force of the flexible element 39 is set by the mechanism 40. In accordance with the value of the preset force the pressure is controlled in the closed hydraulic system which ensures a local increase in the rigidity of the stand. For example, if the force of the piston 35 grows, the movable piston 37 of the control valve 38 is shifted to the right, pressure is fed into the hydraulic cylinder 41 thrusting apart the supports of the backup rolls. Owing to the increase in the thrust of the back-up rolls, pressure in the hydraulic power cylinders 6 drops, thus restoring the equilibrium between the forces applied to the movable piston 37 of the control valve 38.

By changing the preset force applied to the piston 37 by the flexible element 39 is is possible to ensure precise correction of the transverse profile of the rolled strip without changing the thickness of said strip along its length which is a substantial advantage of the present device for automatic thickness control of the rolled strip.

In the electromechanical form of this channel the signal proportional to AP, controls the electric motor 44 which moves the piston 43 in the hydraulic cylinder 42. In case of a positive value of P,, the piston 43 moves to the left until the value of AP, diminishes to zero (or to the preset insensitivity zone-of the channel) If a negative value of AP, appears at the output of the amplifier 26, the piston 43 moves in the reverse direction.

The working roll thrust control channel considered above can be realized so that the output signal of the amplifier 26 will act directly on the electric drive 5 -of the pressure screws 4. In this case it is practicable that an insensitivity zone should be set for this electric drive 5.

The roll stand prestressing circuit combined with said channels of positive feedback and working roll thrust control ensures precise correction of all the actual disturbances affecting the thickness of the strip except for the machining inaccuracy of the working rolls. This inaccuracy is one or more orders smaller than the machining inaccuracy of the back-up rolls and is the only cardinal limitation of accuracy in strip thickness control. To eliminate this limitation, the device is provided with a circuit for programmed correction of the influence of working roll machining inaccuracy on the thickness of the strip. On principle, such correction is possible with relation to the working rolls because the working rolls of a roll stand are interconnected by a gear stand (not shown in the drawing) so that the runout of the working rolls is strictly regular,,repeating during each revolution of the roll. The relative angular position of the working rolls is measured by a pulse transmitter of roll rotation. A marking pulse continuously indicates the beginning of counting of the angular position of the rolls during each revolution.

Before rolling, the working rolls are pressed to each other with a force approaching the rolling pressure, and rotated. The pressure meter 23 reads the variations of pressure in the closed hydraulic system caused by the run-out of the working rolls (the run-out of the back-up rolls is absorbed in this case by the stand bed). These pressure variations are transmitted through the control unit 48 into the memory unit 49 which memorizes said variations as a function of the relative angular position of the working rolls.

In the course of strip rolling the electrical connection between the pressure meter 23 and the memory unit 49 is interrupted. In this case the values of pressure as a function of the relative angular position of the working rolls are fed from the unit 49 into the code-voltage converter 51 whose output signal acts via the amplifier 52 on the electromechanical means 53 which converts electrical voltage into motion. This moves the piston 54 of the correcting hydraulic cylinder 55 so as to make up for the effect of the run-out of the working rolls on the strip thickness.

If it is desired to actuate the device for controlling the strip thickness by a computer, it is practicable that said device be provided with devices setting the volume of the fluid in the closed hydraulic system by individual electric-powered hydraulic cylinders controlled by a computer. In this case it is not required to open the valve 11 for readjusting the stand. Simultaneously, it is practicable that this channel should introduce corrections in the volume of fluid contained in the closed hydraulic system. On account of the above considerations, the device for controlling the thickness of the roll strip according to the invention has the following advantages:

a. there are no limitations in the accuracy of thickness control owing to correction of the disturbances caused by the stand, e.g., run-out of backup rolls, variations in the height and rigidity of the oil wedge in the bearings of the back-up rolls, and run-out of the working rolls;

b. it is not necessary to use high quality dynarnometers because it is possible in this device to dispense with the measurement of the full rolling pressure and sub sequent determination of the pressure increments and to substitute it by direct measurement of the increments (i.e., variations of pressure in the closed hydraulic system);

0. replacement of powerful quick-response actuating mechanisms by relatively low-power mechanisms whose operation makes it possible to provide the necessary insensitivity zone in the operation of powerful actuating mechanism (i.e., pressure screws);

d. the effect of errors in the operation of the actuating mechanisms on the thickness of the strip is reduced by providing the stand prestressing circuit;

e. the realization of the device on the existing mills does not require substantial refitting and long out-ofservice periods.

What we claim is:

1. A device for automatic thickness control of rolled strips in installations manufacturing strip materials and having at least one roll stand with back-up and working rolls, comprising: symmetrically located at each side of said roll stand at least one hydraulic power cylinder installed between the supports of said working rolls; a damping hydraulic cylinder; a piston of said damping hydraulic cylinder, fixed while the rolled strip is passing through the roll stand; a controllable valve, said hydraulic power cylinder, damping cylinder and controllable valve being so connected that they form a closed hydraulic system which ensures a local increase in the rigidity of the roll stand within the range of forces which is approximately equal to the range of rolling pressure deviations from the preset valve; a positive feedback circuit actuated by the fluid pressure in said closed hydraulic system comprising a pressure meter in said hydraulic power cylinder and communicating hydraulically with the latter, and a comparison element for comparing the measured deviation of pressure with the value which is proportional to the preset coefficient of positive feedback, connected with said pressure meter, an actuating element connected to said comparison element, and a correcting element made in the form of a hydraulic cylinder communicating hydraulically with said hydraulic power cylinder and mechanically with said actuating element; when pressure in said closed hydraulic system deviates from the preset value which is caused by the corresponding deviation of the pressure exerted by the rolled strip on the working rolls, the piston of said correcting hydraulic cylinder moves in the direction of the increased deviation of pressure in said closed hydraulic system as a result of a signal produced at the output of said comparison element, said signal acting via said actuating element on said correcting element until the value proportional to the preset coefficient of positive feedback and to the movement of the piston counterbalances the value of pressure deviation. i

2. A device according to claim 1, wherein its positive feedback circuit is of the hydromechanical type and comprises a hydraulic cylinder serving as an actuating element; a control valve whose space is in hydraulic communication with said hydraulic cylinder; a second hydraulic cylinder serving as a pressure meter and whose piston is linked kinematically with one of the ends of the piston of said control valve; a flexible element connected to the other end of the piston of said control valve and linked kinematically with the piston of said correcting hydraulic cylinder.

3. A device according to claim 1, wherein the positive feedback circuit is of the electromechanical type and comprises a divider functioning as a setter of the positive feedback coefficient and connected to said pressure meter and to said comparison element, an

' electromechanical means for converting electrical volthydraulic cylinders installed between each pair of sup- 7 ports of said back-up rolls.

6. A device according to claim 4, wherein said working roll thrust control channel controls the electrical drive of the pressure screws.

7. A device according to claim 5, wherein said working roll thrust control channel comprises a working roll thrust setter; a pressure meter for measuring pressure in the hydraulic power cylinder; a control valve hydraulically communicating with the hydraulic cylinder installed between the supports of the back-up rolls; a movable piston of said control valve one end of which is kinematically connected with said pressure meter of the thrust control channel while its other end is connected to said working roll thrust setter.

8. A device according to claim 5, wherein said working roll thrust control channel comprises a correcting element in the form of a hydraulic cylinder communicating hydraulically with the hydraulic cylinder installed between the supports of said back-up rolls; an-

electromechanical means for converting electrical voltage into motion, mechanically connected with the piston of said correcting hydraulic cylinder and electrically connected with said pressure meter .of said positive feedback channel.

9. A device according to claim. 1, which has a channel for programmed correction of working roll machining inaccuracies, this channel comprising: a memory unit for memorizing the variations of pressure in said hydraulic power cylinder as a function of the turning angle of the working rolls; a unit for measuring the current values of the working roll turning angle; an electromechanical means for converting electrical voltage into motion, connected electrically to said memory unit and to the unit for measuring current values of turning angles; one more correcting hydraulic cylinder hydraulically connected with said hydraulic power cylinder, the piston of the correcting cylinder being coupled with said electromechanical means of the programmed correction channel; a control unit for controlling said memory unit and current value measuring unit which ensures synchronization of the action of said units on said electromechanical conversion means in the programmed correction channel. 

1. A device for automatic thickness control of rolled strips in installations manufacturing strip materials and having at least one roll stand with back-up and working rolls, comprising: symmetrically located at each side of said roll stand at least one hydraulic power cylinder installed between the supports of said working rolls; a damping hydraulic cylinder; a piston of said damping hydraulic cylinder, fixed while the rolled strip is passing through the roll stand; a controllable valve, said hydraulic power cylinder, damping cylinder and controllable valve being so connected that they form a closed hydraulic system which ensures a local increase in the rigidity of the roll stand within the range of forces which is approximately equal to the range of rolling pressure deviations from the preset value; a positive feedback circuit actuated by the fluid pressure in said closed hydraulic system comprising a pressure meter in said hydraulic power cylinder and communicating hydraulically with the latter, and a comparison element for comparing the measured deviation of pressure with the value which is proportional to the preset coefficient of positive feedback, connected with said pressure meter, an actuating element connected to said comparison element, and a correcting element made in the form of a hydraulic cylinder communicating hydraulically with said hydraulic power cylinder and mechanically with said actuating element; when pressure in said closed hydraulic system deviates from the preset value which is caused by the corresponding deviation of the pressure exerted by the rolled strip on the working rolls, the piston of said correcting hydraulic cylinder moves in the direction of the increased deviation of pressure in said closed hydraulic system as a result of a signal produced at the output of said comparison element, said signal acting via said actuating element on said correcting element until the value proportional to the preset coefficient of positive feedback and to the movement of the piston counterbalances the value of pressure deviation.
 2. A device according to claim 1, wherein its positive feedback circuit is of the hydromechanical type and comprises a hydraulic cylinder serving as an actuating element; a control valve whose space is in hydraulic communication with said hydraulic cylinder; a second hydraulic cylinder serving as a pressure meter and whose piston is linked kinematically with one of the ends of the piston of said control valve; a flexible element connected to the other end of the piston of said control valve and linked kinematically with the piston of said correcting hydraulic cylinder.
 3. A device according to claim 1, wherein the positive feedback circuit is of the electromechanical type and comprises a divider functioning as a setter of the positive feedback coefficient and connected to said pressure meter and to said comparison element, an electromechanical means for converting electrical voltage into motion with negative feedback actuated by said motion, connected to said comparison element and linked with the piston of said correcting hydraulic cylinder.
 4. A device according to claim 1, comprising a channel for controlling the thrust of saId working rolls, said channel stabilizing the thrust with relation to the preset value.
 5. A device according to claim 4, wherein said working roll thrust control channel controls the pressure in hydraulic cylinders installed between each pair of supports of said back-up rolls.
 6. A device according to claim 4, wherein said working roll thrust control channel controls the electrical drive of the pressure screws.
 7. A device according to claim 5, wherein said working roll thrust control channel comprises a working roll thrust setter; a pressure meter for measuring pressure in the hydraulic power cylinder; a control valve hydraulically communicating with the hydraulic cylinder installed between the supports of the back-up rolls; a movable piston of said control valve one end of which is kinematically connected with said pressure meter of the thrust control channel while its other end is connected to said working roll thrust setter.
 8. A device according to claim 5, wherein said working roll thrust control channel comprises a correcting element in the form of a hydraulic cylinder communicating hydraulically with the hydraulic cylinder installed between the supports of said back-up rolls; an electromechanical means for converting electrical voltage into motion, mechanically connected with the piston of said correcting hydraulic cylinder and electrically connected with said pressure meter of said positive feedback channel.
 9. A device according to claim 1, which has a channel for programmed correction of working roll machining inaccuracies, this channel comprising: a memory unit for memorizing the variations of pressure in said hydraulic power cylinder as a function of the turning angle of the working rolls; a unit for measuring the current values of the working roll turning angle; an electromechanical means for converting electrical voltage into motion, connected electrically to said memory unit and to the unit for measuring current values of turning angles; one more correcting hydraulic cylinder hydraulically connected with said hydraulic power cylinder, the piston of the correcting cylinder being coupled with said electromechanical means of the programmed correction channel; a control unit for controlling said memory unit and current value measuring unit which ensures synchronization of the action of said units on said electromechanical conversion means in the programmed correction channel. 